1. Field of the Invention
This invention relates to conductive polymer compositions and electrical devices comprising them.
2. Introduction to the Invention
Conductive polymer compositions and electrical devices comprising them are well-known. Reference may be made, for example, to U.S. Pat. Nos. 4,188,276 (Lyons et al), 4,237,441 (van Konynenburg et al), 4,238,812 (Middleman et al), 4,317,027 (Middleman et al), 4,352,083 (Middleman et al), 4,388,607 (Toy et al), 4,413,301 (Middleman et al), 4,534,889 (van Konynenburg et al), 4,545,926 (Fouts et al), 4,560,498 (Hormsa et al), 4,935,156 (van Konynenburg et al), and 5,049,850 (Evans et al), and copending, commonly assigned application Ser. Nos. 07/75,929 (Barma et al, filed Jul. 21, 1987), now U.S. Pat. No. 5,106,540, issued Apr. 21, 1992, 07/114,488 (Blake et al, filed Oct. 28, 1987), and 07/462,893 (Soni et al, filed Jan. 3, 1990), the disclosures of which are incorporated herein by reference.
For many applications such as circuit protection devices and self-regulating heaters it is desirable that the conductive polymer composition exhibit positive temperature coefficient of resistance behavior (PTC), i.e. that the resistance of the composition increase anomalously as a function of temperature. Conventionally, conductive polymer compositions exhibiting PTC behavior have been made by mixing particulate conductive fillers with a crystalline polymer. For these materials, the temperature at which the resistance anomalously increases is dependent on the melting point of the polymer, i.e. the material generally "switches" into its high resistance state at a temperature, T.sub.s, which is slightly below the melting point of the polymer. As a result, the range of available switching temperatures is limited by the melting points of available crystalline polymers.
U.S. Pat. No. 4,534,889 (van Konynenburg et al) describes a process for producing PTC behavior in a relatively noncrystalline elastomer by crosslinking the elastomer, in which conductive particles are dispersed, at a temperature T.sub.c. If a sufficient level of crosslinking is imparted, the elastomer will exhibit PTC behavior at a temperature around T.sub.c. However, because the matrix polymer is an elastomer, such materials may suffer from resistance instability when exposed to repeated thermal cycles or voltage applications.
U.S. Pat. No. 4,966,729 (Carmona et al) discloses a material in which conductive fibers are dispersed in a thermosetting polymer matrix, e.g. an epoxy or a silicone, in order to produce a material which has improved thermal stability and which can be mixed at room temperature. Following annealing, the material exhibits PTC behavior. Such materials, which contain relatively little conductive fiber, e.g. 1 to 3% by volume, are subject to nonuniform mixing, and settling of the fibers during annealing.